A portable user device may rely on a global navigation satellite system (GNSS) to determine its position. However, there are some venues, referred to as GNSS dead zones, in which the user device cannot receive communication signals from the GNSS satellites, such as in the interior of radio-opaque buildings. The GNSS functionality therefore provides no help in determining the position of the user device as it moves within such an environment. The industry has proposed alternative techniques to determine the location of the user device within GNSS dead zones. But there remains room for improvement regarding the manner in which these alternative techniques are invoked.